The present invention relates generally to focused ultrasound systems and, more particularly, to systems and methods for controlling a phased array transducer in a focused ultrasound system in order to focus acoustic energy transmitted by respective transducer elements at one or more target focal zones in a patient""s body.
High intensity focused acoustic waves, such as ultrasonic waves (i.e., with a frequency greater than about 20 kilohertz), may be used to therapeutically treat internal tissue regions within a patient. For example, ultrasonic waves may be used to ablate tumors, eliminating the need for invasive surgery. For this purpose, focused ultrasound systems having piezoelectric transducers driven by electric signals to produce ultrasonic energy have been employed.
In systems, such as a focused ultrasound system, the transducer is positioned external to the patient, but in generally close proximity to a target tissue region within the patient to be ablated. The transducer may be geometrically shaped and positioned so that the ultrasonic energy is focused at a xe2x80x9cfocal zonexe2x80x9d corresponding to the target tissue region, heating the region until the tissue is necrosed. The transducer may be sequentially focused and activated at a number of focal zones in close proximity to one another. For example, this series of xe2x80x9csonicationsxe2x80x9d may be used to cause coagulation necrosis of an entire tissue structure, such as a tumor, of a desired size and shape.
By way of illustration, FIG. 1 depicts a phased array transducer 10 having a xe2x80x9cspherical capxe2x80x9d shape. The transducer 10 includes a plurality of concentric rings 12 disposed on a curved surface having a radius of curvature defining a portion of a sphere. The concentric rings 12 generally have equal surface areas and may also be divided circumferentially 14 into a plurality of curved transducer sectors, or elements 16, creating a xe2x80x9ctilingxe2x80x9d of the face of the transducer 10. The transducer elements 16 are constructed of a piezoelectric material such that, upon being driven with a sinus wave near the resonant frequency of the piezoelectric material, the elements 16 vibrate according to the phase and amplitude of the exciting sinus wave, thereby creating the desired ultrasonic wave energy.
As illustrated in FIG. 2, the relative phase shift and amplitude of the sinus drive signal for each transducer element 16 is individually controlled so as to sum the emitted ultrasonic wave energy 18 at a focal zone 13 having a desired focused planar and volumetric pattern. This is accomplished by coordinating the signal phase of the respective transducer elements 16 in such a manner that they constructively interfere at specific locations, and destructively cancel at other locations. For example, if each of the elements 16 are driven with drive signals that are in phase with one another, (known as xe2x80x9cmode 0xe2x80x9d), the emitted ultrasonic wave energy 18 are focused at a relatively narrow focal zone. Alternatively, the elements 16 may be driven with respective drive signals that are in a predetermined shifted-phase relationship with one another (referred to in U.S. Pat. No. 4,865,042 to Umemura et al. as xe2x80x9cmode nxe2x80x9d). This results in a focal zone that includes a plurality of 2n zones disposed about an annulus, i.e., generally defining an annular shape, creating a wider focus that causes necrosis of a larger tissue region within a focal plane intersecting the focal zone. Various distances, shapes and orientations (relative to an axis of symmetry) of the focal zone can be created by controlling the relative phases and amplitudes of the emitted energy waves from the transducer array, including steering and scanning of the beam, thereby enabling electronic control of the focused beam to cover and treat multiple spots in a target tissue area (e.g., a defined tumor) inside the patient""s body.
More advanced techniques for obtaining specific focal zone characteristics are disclosed in U.S. patent application Ser. No. 09/626,176, filed Jul. 27, 2000, entitled xe2x80x9cSystems and Methods for Controlling Distribution of Acoustic Energy Around a Focal Point Using a Focused Ultrasound System;xe2x80x9d U.S. patent application Ser. No. 09/556,095, filed Apr. 21, 2000, entitled xe2x80x9cSystems and Methods for Reducing Secondary Hot Spots in a Phased Array Focused Ultrasound System;xe2x80x9d and U.S. patent application Ser. No. 09/557,078, filed Apr. 21, 2000, entitled xe2x80x9cSystems and Methods for Creating Longer Necrosed Volumes Using a Phased Array Focused Ultrasound System.xe2x80x9d The foregoing patent applications, along with U.S. Pat. No. 4,865,042, are all hereby incorporated by reference for all they teach and disclose.
It is significant to implementing these focal zone positioning and shaping techniques to provide a transducer control system that allows the phase of each transducer element to be independently controlled. To provide for precise positioning and dynamic movement and reshaping of the focal zone, it is desirable to be able to alter the phase and/or amplitude of the individual elements relatively fast, e.g., in they second range, to allow switching between focal zone characteristics or modes of operation. As taught in the above-incorporated U.S. patent application Ser. No. 09/556,095, it may also be desirable to be able to rapidly change the drive signal frequency of one or more elements. In a MRI-guided focused ultrasound system, it is desirable to be able to drive the ultrasound transducer array without creating electrical harmonics, noise, or fields that interfere with the ultra-sensitive receiver signals that create the images.
Thus, it is desirable to provide a system and methods for individually controlling, and dynamically changing, the driving voltage, phase and amplitude of each transducer element in phased array focused ultrasound transducer a manner that does not interfere with the imaging system.
The present invention provides systems and methods for controlling the phase and amplitude of individual drive sinus waves of a phased-array focused ultrasound transducer. In one embodiment, digital potentiometers are used to scale the amplitude of a selected two of four orthogonal bases sinuses having respective phases of 0xc2x0, 90xc2x0, 180xc2x0, and 270xc2x0 into component sinus vectors. The component sinus vectors are linearly combined to generate the respective sinus of a selected phase and amplitude. The use of digitally controlled potentiometers allows for digitally controlled switching between various focal zone characteristics. For example, the respective input parameters for any number of possible focal zone distances, shapes and orientations may be stored in a comprehensive table or memory for readily switching between the various focal zone characteristics in xcexc seconds.
In a preferred embodiment, changes in the output frequency are also readily accomplished without impacting on the specific focal zone characteristics of the transducer output. Towards this end, sequential changes in the distance, shape and/or orientation of the focal zone are implemented in the form of sequential sets of digital control signals (or xe2x80x9csonication parametersxe2x80x9d) transmitted from the central controller to respective control channels for generating the individual sinus waves. The digital control signals may be changed in accordance with a time-domain function as part of a single thermal dose, or xe2x80x9csonication.xe2x80x9d In other words, during a single sonication, the systems and methods provided herein allow for switching between ultrasound energy beam focal shapes and locations at a rate that is relatively high compared to the heat transfer time constant in a patient""s tissue.
In accordance with a further aspect of the invention, each set of sonication input parameters has a corresponding set of expected, or planned, output phase and amplitude levels for each sinus wave. The actual output levels are then measured and if either of the actual phase or amplitude differs from what is expected for the respective sinus wave, the particular drive sinus wave, or perhaps the entire system, may be shut down as a precautionary safety measure.
Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.